This invention relates to cathode ray tubes (CRTs) for color television and other display applications, and more particularly relates to the shadow mask which is employed to obtain registration of the electron beams from the electron gun with the phosphor elements in the display screen.
Most CRTs for color television and other display applications, such as computer monitors, employ a shadow mask, a thin metal sheet having a very large number of apertures, to achieve registration of the scanning electron beams from the electron gun with the cathodoluminescent phosphor elements which constitute the display screen.
The apertures are sized and positioned to allow only a portion of each beam to pass through and land on the correct phosphor element. While some portion of the beam not passing through an aperture is scattered, a much larger portion is absorbed by the mask material and converted to heat. Due to the low mass and high thermal expansion coefficient of the thin metal mask, it heats up and expands at a faster rate than surrounding structures, causing the mask to "dome", ie, move closer to the screen, causing misregistration between the electron beams and the screen. Such misregistration lowers resolution and color purity of the display. Such doming is most severe during the initial warm up phase of operation, but can also occur during normal tube operation, caused for example, by local areas of high intensity in the display image. The latter effect is referred to as "local doming".
The trend toward higher resolution in the tv industry, and the higher resolution requirements of the computer industry in general, has led to efforts to minimize doming in such color CRTs. One approach has been to replace the traditional steel masks with masks made from invar, an alloy of 36% by weight nickel, remainder iron and impurities, because such masks have a lower coefficient of thermal expansion than do steel masks. However, invar is a more expensive material than steel. Moreover, invar has a higher yield stress, and must be formed at elevated temperatures. In addition, invar masks, due to their lower modulus of elasticity, have a greater tendency to buckle under mechanical shock than do steel masks. These factors all lead to an overall increase in the cost of the CRT.
Japanese patent application JP 62-82630 teaches forming a shadow mask from a composite material consisting of an inner core of an invar sheet sandwiched between two outer steel sheets. The volume ratio between the outer steel layers and the inner invar layer is kept within the range of 0.15 to 0.50, below which the modulus of elasticity is said to deteriorate too much, and above which the coefficient of thermal expansion is said to be excessive.
Japanese patent application JP 62-82631 teaches forming a shadow mask from a composite material consisting of an inner core of a steel sheet between two outer invar sheets. The volume ratio between the inner steel layer and the outer invar layers is kept within the range of 0.15 to 0.50, below which the modulus of elasticity is said to deteriorate too much, and above which the coefficient of thermal expansion is said to be excessive.
A table common to both Japanese applications presents values for average thermal expansion coefficient, 0.2% proof stress and elasticity coefficient, for different values of the ratio of the inner to outer layers of the composites. It can be calculated that in both composite structures, invar ranges from 67% to 87% and steel ranges from 13% to 33% volume %.
Such three-layer composites tend to be difficult and costly to fabricate, particularly because of the high percentage of invar present. Furthermore, such a high percentage of invar means that the 0.2% proof stress of these composites is still in the range which requires forming of the shadow masks at elevated temperatures.